1. Field of the Invention
The present invention relates to a honeycomb structure, particularly to a honeycomb structure which is preferably usable as a filter for purifying an exhaust gas or a catalyst carrier.
2. Description of the Related Art
In recent years, influences of particulate matter or NOx discharged from an engine for an automobile, especially a diesel engine or the like onto environments have been remarkably brought into the public eye, and the use of a honeycomb structure including porous partition walls has variously been studied as important means for removing these noxious materials.
For example, a honeycomb structure for a filter (hereinafter referred to as DPF) for filtering the particulate matter discharged from the diesel engine has been developed. The honeycomb structure for DPF generally includes porous partition walls disposed so as to form a plurality of cells extending in an axial direction, and the cells disposed adjacent to each other via the partition wall are plugged in end faces opposite to each other. Moreover, an exhaust gas is introduced into the cell which opens in one end face, and is passed through the partition wall in the honeycomb structure, so that the particulate matter in the exhaust gas can be filtrated and removed (see JP-A-4-301114).
For such a honeycomb structure, gas flows of central and outer peripheral portions in a vertical section with respect to the axial direction (longitudinal direction of the cell) are desired to be uniform from the standpoints of enhancement of efficiency of the filter and reduction of pressure loss. It is also necessary to periodically burn and remove the particulate matter deposited on the partition wall and to regenerate the filter, and a catalyst used for such a purpose is also desired to be quickly activated as needed. At the time of the burning of the particulate matter, the generation of cracks by a thermal stress caused by a higher temperature at the central portion sometimes raises a problem, and it has also been desired that the cracks are not easily generated.
To prevent the cracks by the thermal stress, there has been proposed a method in which a plurality of segments obtained by dividing a honeycomb filter are bonded by a bonding material. For example, there is disclosed a ceramic honeycomb structure obtained by bonding honeycomb ceramic members with an elastic sealant made of at least a three-dimensionally intertwined inorganic fiber, an inorganic binder, an organic binder and inorganic particles (see JP-A-8-28246).
However, when the filter is segmented and bonded, an extra step is required, and further the presence of a bonding layer that does not function as the filter lowers the efficiency of the filter. Therefore, there has been a demand for a honeycomb structure which can meet the above-described requirement, even when not segmented.